A direct conversion receiver is being studied very actively as one of receivers for embodying single chip. The direct conversion receiver can reduce the number of outer devices such as a filter and can relieve burden of digital signal processing. Thus, it is the most suitable device for manufacturing a single chip using CMOS process where digital circuit can be easily conceived. The direct conversion receiver comprises a RF direct conversion receiver for converting RF (Radio Frequency) signal into base-band and an IF direct conversion receiver for converting RF signal into IF (Intermediate Frequency) signal and for converting the RF signal into base-band.
FIG. 1 shows a block diagram of a conventional vector RF direct conversion receiver embodied in CMOS process.
As shown in FIG. 1, the vector RF direct conversion receiver comprises a band pass filter (101), a low noise amplifier (103), a phase conversion device (105), first and second active mixers (107, 109), and base-band analog circuit (111). Low noise amplifier (103), first and second active mixers (107, 109) and base-band analog circuit (111) are embodied by CMOS process. The direct conversion receiver shown in FIG. 1 outputs two vector base-band signals of I and Q as a result of mixing radio frequency signal with phase local oscillating signal and quadrature phase local oscillating signal.
FIG. 2 shows a block diagram of a conventional vector IF direct conversion receiver embodied in CMOS process.
As shown in FIG. 2, the vector IF direct conversion receiver comprises a band pass filter (201), a low noise amplifier (203), a first active mixer (205), a phase conversion device (207), a second and a third active mixer (209, 211) and base-band analog circuit (213). Low noise amplifier (203), first, second and third active mixers (205, 209, 211) and base-band analog circuit (213) are embodied by CMOS process. The IF direct conversion receiver shown in FIG. 2 converts radio frequency signal into intermediate frequency signal by first active mixer (205), which is converted into two vector base-band signals of I and Q to be outputted.
FIG. 3 shows a circuit diagram of a typical mixer embodied using CMOS Gilbert cell in the direct conversion receiver shown in FIGS. 1 and 2.
As shown in FIG. 3, the mixer comprises an amplifying unit (3100) and a mixing unit (3300). Amplifying unit (3100) comprises amplifying device MA31 and amplifies the inputted signal. Mixing unit (3300) comprises a first and second switching devices MS31 and MS32; mixes an input signal and local oscillating signal LO; and outputs a signal corresponding to the frequency difference between the two signals. In the conventional CMOS direct conversion receiver, the amplifying device MA31, first and second switching devices MS31 and MS32 are embodied with MOS devices.
The conventional direct conversion receiver is difficult to be embodied as an integrated circuit because of DC offset due to local oscillator leakage and mismatching between I/Q circuits. In particular, the following problems exist in the event that the direct conversion receiver is embodied only in CMOS process as shown in FIG. 1.
First, there are additional DC offset and system noise figure degradation due to mis-matching between MOS devices and l/f noise within devices. In particular, the first and second switching devices MS31 and MS32 shown in FIG. 3 and MOS device used in base-band analog circuits (111), all of which output low frequency base-band signal, are significant causes for the above problems. These problems cannot be resolved completely. Wide-band wireless system can resolve the problem to some extent by use of high pass filer. However, in narrow-band system wherein signal bandwidth is less than l/f noise corner frequency, there are severe degradation of noise ratio with respect to signal and severe reduction in system dynamic range. Further, the circuit cannot operate completely as result of saturation.
Second, mismatching between the signal paths of I and Q occurs due to mis-matching between MOS devices. This causes degradation of noise ratio with respect to signal.
Bipolar junction transistor (BJT) has good matching characteristic between devices compared with the MOS devices and has l/f noise less than 1/100 l/f noise of MOS devices. Thus, the bipolar junction transistor can resolve the DC offset and system noise characteristic degradation due to l/f noise to a great extent. Thus, a direct conversion receiver using BiCMOS process, where CMOS devices and BJT devices are integrated together, was developed. The direct conversion receiver using BiCMOS process has improved DC offset and l/f noise characteristic compared with the receiver using MOS process. However, it is expensive and it takes long time to develop the receiver. Further, digital circuit performance is very bad compared with the receiver using CMOS process. Thus, the direct conversion receiver using BiCMOS process is disadvantageous in embodying a single chip.
On the other hand, a research has been conducted for resolving the problems of the MOS devices using side BJT available only in the CMOS process or vertical parasitic BJT. However, the BJT devices have very bad operation frequency performance compared with the MOS device, thereby limiting its use in DC circuits such as bandgap reference. In particular, the side BJT has a disadvantage in matching characteristic between devices compared with vertical BJT.